Sump (cave)

When short in length, a sump may be called a duck, however this can also refer to a section or passage with some (minimal) airspace above the water.

This is known as "free diving" and can only be attempted if the sump is known to be short and not technically difficult (e.g. constricted or requiring navigation).

Upstream sumps have been successfully emptied using hoses to siphon water out of them, such as at the Sinkhole Dersios during exploration in 2005.

During a rescue from beyond a downstream sump at Sarkhos Cave in 2002, water was pumped upstream into a dam constructed a few metres above the flooded passage.

Some manuals also mention the use of explosives or other forms of force to empty sumps, but the ecological damage done to the fragile cave environment usually rules out the use of such methods.

Two sectional diagrams illustrating the concept of a sump. Diagram "A" illustrates a U-shaped passage with water filling the rounded bottom section, blocking the dry passage either side. Diagram "B" shows a passage blocked similarly by a sump, but on one side the water level is being held back by a natural dam, with dry passage continuing beyond, below the water level of the sump.
Sumps often block access to "dry" passage beyond them. Diagram B shows a "perched" sump, which could be siphoned to lower the water level.